Warming, interrupted: Much ado about natural variability

A guest commentary by Kyle Swanson – University of Wisconsin-Milwaukee

I am quite humbled by the interest that has been generated by our paper “Has the climate recently shifted?” (Swanson and Tsonis, 2009), and would like the thank the RealClimate editors for the opportunity to give my perspective on this piece.

Before delving into the paper itself, a few words about the place of our work in the global warming “debate” are in order. A quote from the early 20th century Viennese polymath Egon Friedell (which I ran across in the wonderful book Cultural Amnesia by Clive James) captures the situation better than any words I could ever weave;

Electricity and magnetism are those forces of nature by which people who know nothing about electricity and magnetism can explain everything.

Substitute the words “modes of natural climate variability” for “electricity and magnetism,” and well…, hopefully the point is made.

It first needs to be emphasized that natural variability and radiatively forced warming are not competing in some no-holds barred scientific smack down as explanations for the behavior of the global mean temperature over the past century. Both certainly played a role in the evolution of the temperature trajectory over the 20th century, and significant issues remain to be resolved about their relative importance. However, the salient point, one that is oftentimes not clear in arguments about variability in the climate system, is that all else being equal, climate variability and climate sensitivity are flip sides of the same coin. (see also the post Natural Variability and Climate Sensitivity)

A climate that is highly sensitive to radiative forcing (i.e., responds very strongly to increasing greenhouse gas forcing) by definition will be unable to quickly dissipate global mean temperature anomalies arising from either purely natural dynamical processes or stochastic radiative forcing, and hence will have significant internal variability. The opposite also holds. It’s painfully easy to paint oneself logically into a corner by arguing that either (i) vigorous natural variability caused 20th century climate change, but the climate is insensitive to radiative forcing by greenhouse gases; or (ii) the climate is very sensitive to greenhouse gases, but we still are able to attribute details of inter-decadal wiggles in the global mean temperature to a specific forcing cause. Of course, both could be wrong if the climate is not behaving as a linear forced (stochastic + GHG) system.

With that in mind, our paper is fundamentally about inter-decadal variability in the climate system and its role in the evolution of the 20th century climate trajectory, as well as in near-future climate change. The climate system has well known modes of variability, such as the El Niño/Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO), that are active on inter-annual time scales. We are interested in how this short time-scale (from the climate perspective!) variability impacts climate anomalies over multi-decadal time periods.

What we find is that when interannual modes of variability in the climate system have what I’ll refer to as an “episode,” shifts in the multi-decadal global mean temperature trend appear to occur. I’ll leave the details of these episodes to interested readers (here and here), as things get pretty technical. It’s sufficient to note that we have an objective criteria for what defines an episode; we aren’t just eyeballing curves. The climate system appears to have had three distinct “episodes” during the 20th century (during the 1910’s, 1940’s, and 1970’s), and all three marked shifts in the trend of the global mean temperature, along with changes in the qualitative character of ENSO variability. We have also found similar types of shifts in a number of model simulations (both forced and unforced) that were run in support of the IPCC AR4 report.

The contentious part of our paper is that the climate system appears to have had another “episode” around the turn of the 21st century, coinciding with the much discussed “halt” in global warming. Whether or not such a halt has really occurred is of course controversial (it appears quite marked in the HadCRUT3 data, less so in GISTEMP); only time will tell if it’s real. Regardless, it’s important to note that we are not talking about global cooling, just a pause in warming.

What’s our perspective on how the climate will behave in the near future? The HadCRUT3 global mean temperature to the right shows the post-1980 warming, along with the “plateau” in global mean temperature post-1998. Also shown is a linear trend using temperatures over the period 1979-1997 (no cherry picking here; pick any trend that doesn’t include the period 1998-2008). We hypothesize that the established pre-1998 trend is the true forced warming signal, and that the climate system effectively overshot this signal in response to the 1997/98 El Niño. This overshoot is in the process of radiatively dissipating, and the climate will return to its earlier defined, greenhouse gas-forced warming signal. If this hypothesis is correct, the era of consistent record-breaking global mean temperatures will not resume until roughly 2020. Of course, this contrasts sharply with other forecasts of the climate system; the purple line roughly indicates the model-based forecast of Smith et al. (2007) , suggesting with a warming of roughly 0.3 deg C over the 2005-2015 period.

Why would anyone in their right mind believe what I’ve just outlined? Everything hinges on the idea that something extraordinary happened to the climate system in response to the 1997/98 super-El Niño event (an idea that has its roots in the wavelet analysis by Park and Mann (2000)). The figure to the left shows the spatial mean temperature over all grid boxes in the HadCRUT3 data set that have continuous monthly coverage over the 1901-2008 period. While this provides a skewed view of the global mean, as it is heavily weighted toward North America, Europe and coastal areas, unlike the global mean temperature it has the cardinal virtue of being a consistent record with respect to time. The sole exclusion in the figure is the line connecting the 1997 and 1998 temperatures.

Now, anomalous behavior is always in the eye of the beholder. However, the jump in temperature between 1997 and 1998 in this record certainly appears to pass the “smell test” (better than 3 standard deviations of interannual variability) for something out of the ordinary. Nor is this behavior dependent on the underlying time interval chosen, as the same basic picture emerges for any starting time up until the 1980’s, provided you look at locations that have continuous coverage over your interval. Again, as the temperature anomaly associated with this jump dissipates, we hypothesize that the climate system will return to its signal as defined by its pre-1998 behavior in roughly 2020 and resume warming.

What do our results have to do with Global Warming, i.e., the century-scale response to greenhouse gas emissions? VERY LITTLE, contrary to claims that others have made on our behalf. Nature (with hopefully some constructive input from humans) will decide the global warming question based upon climate sensitivity, net radiative forcing, and oceanic storage of heat, not on the type of multi-decadal time scale variability we are discussing here. However, this apparent impulsive behavior explicitly highlights the fact that humanity is poking a complex, nonlinear system with GHG forcing – and that there are no guarantees to how the climate may respond.

So I await some convincing conviction statement on this temperature pause. Seems that current El-Nino is poised to surpass 1998 one, if this trend holds, Global temperatures may be highest in history soon.

In the discussions of global warming and global cooling, are we confusing heat with temperature? The annual mean land and sea surface TEMPERATURES (which CAN be measured)appear to be being used as a proxy for the total HEAT of the global atmospheric and oceanic system (which cannot be measured directly). But could you not have a significant increase in the total heat gained by the system at the same time that the GISS and Hadcrut land/SST measurements are coming in slightly lower?

For example, in a strong La Nina as we have just had, if colder water is being drawn up to the ocean surface, it would also transfer heat from the lower atmosphere to lower waters. But the SST would still be lower than normal. Would this lower temp. at the surface result in lower radiation out from the surface to the upper atmsophere, resulting in less heat lost to space and more heat retained by the system?

Also, if there were to be a significant amount of melting of land glaciers and Greenland and WAIS ice in the same year, could the higher amounts of heat energy held in by GHGs be “hidden” by the heat of fusion in converting solid to liquid water? In other words, is it possible that during La Ninas and/or times of significant planetary ice loss, the surface temps would be going down while the total heat energy of the oceans is going up, with climatic results to become apparent later?

You identified a breakpoint in the dataset at 1997. I analysed the HadCRUt3 simplified dataset (monthly data http://hadobs.metoffice.com/crutem3/diagnostics/global/simple_average/monthly) and identified three breakpoints in the dataset one at 1977, one at 1986 and the breakpoint at 1997. This was using the strucchange package in R. The 1977 breakpoint can be explained by a paper by McGuirk (Planetary-Scale Forcing of the January 1977 Weather, Science 199:4326 293-295 1978). The 1986 and 1997 breakpoints both coincide with El Nino events.

My hypothesis is that each of these are cumulative adding one on the other. Much of the observed warming from 1977 through to now may be a result of this cumulative effect where the next event caused more heat to be added to the system before the heat from the previous event radiatively dissipated.

But even if you had satellites in the right orbit, getting long-term instrumental stability to a level where you could detect the relatively small imbalances caused by ocean/atmosphere heat exchange would be difficult. Hansen and others argue that direct in situ monitoring of ocean heat storage, while also difficult, is less so.

I can’t help but feel they are influenced by the fact that the same instruments needed for widespread direct in situ monitoring of ocean heat storage would also provide a great deal of information about ocean currents, the numerous ocean-driven climate modes (like ENSO), and in general a great deal of information against which test climate models.

So, Mr. Swanson, in short natural systems work in synergy and will in fact effectively reduce and pause warming; ocean/atmospheric dynamics convert energy continually, seems elementary to believe as you pointed out astutely that such digressions are reactions/responses in/on the system. General Chem and environmental science tells us this is so; this is a good review–educationaly post! People please re-read and the mod responses.

[Response: This is not at all what the article says. The “pause,” if it indeed exists, is temporary and has little if any effect on long-term climate sensitivity. You are just bringing your own preconceptions and misconceptions into it, and you don’t seem to know either your General Chem or environmental science very well. Systems do tend toward a new equilibrium when conditions change, but the displacement of that equilibrium from the old one can be either amplified or damped by feedbacks. And sometimes, the equilibrium represents a transition to a new state altogether and in essence destroys the old state — as in what happens when you pass the transition temperature for initiating combustion. There, that make you feel better? –raypierre]

#106 (raypiere) Absolutley! The thermohaline, and other oceanic dynamics are of utmost importance to consider. The Earth consists of a dynamic system, and in equilibrium it is dynamic and not static or 100%

I have decided to change my name from steve to stevec to help avoid any possible confusion over which are my posts. I have read comments that there is a lag time between surface temperatures such as measured by GISS and tropospheric temperatures such as measured by RSS and wondered if there was an established correlation and what might this time period be. thanks

The problem with current debate is that it focuses on global mean, annual temperatures (as dictated by climate models). You can put heat into a system, however, WITHOUT CHANGING THE TEMPERATURE. You can add heat to water at its boiling point with the temperature remaining the same (at 100C). As we all know, extra heat added changes the state of matter (liquid to gas). A green house gas molecule will absorb a photon of ir radiation without a detectable temperature increase – if the absorption line is sharp and discrete. The extra heat is used to change the molecule to a new vibrational state.

Even excluding other factors therefore, like solar activity, it should be of no surprise that as we add more heat to the atmosphere, by increasing green house gas concentrations, annual mean global temperatures may not always increase accordingly. Other processes may be occurring that use up this extra heat – in ways that may even be unknown in such a complex system.

Extra heat that accumulates in the atmosphere is redistributed in space (around the globe) and time (throughout the year and different seasons – when local conditions vary). I agree, therefore, with a view expressesd to me by Roger Pielke – that the focus should be on heat itself (and not temperature) and on spatial (local) and seasonal variability. The trends might then be more consistent and less open to doubt and criticism. It might also demonstrate that climate changes are already occurring on a local and seasonal basis.

In a simple analysis of temperature records in a part of mid – Wales carried out in 2006, mean mid – winter and mid – summer temperature were seen to have increased by around 0.9 degrees Celsius over 60 years. Mean, spring time temperatures, however, had decreased slightly over the same period. The focus on mean annual global temperatures completely misses something like this – and this is far more meaningful and of significance to people living locally.

The various “crystal ball” estimates of what is likely to happen to our climate once the current warming pause has ended are interesting.

Rather than picking a trend starting in 1979 or 1950, maybe it would make more sense to take a longer-term look. After all, climate change is not something that can be measured using only short-term “blips” of a few decades in the record.

If we take the entire Hadley record from 1850 to 2008, we see that the linear trend line is represented by the equation:
y = 0.0041x – 0.4965

This represents a long-term linear average warming rate of 0.041°C per decade.

Continuing the long-term trend to year 2050, we arrive at a temperature anomaly of 0.328°C, which is quite close to the present 2008/09 value.

So this would mean that the warming pause would continue until around year 2050, at which point warming would again follow the long-term trend line.

At this continued warming rate we would have reached 0.536°C by year 2100, or slightly higher than the previous all-time record El Nino year of 1998.

Max Acker (manacker) draws a trend from 1850 forward in temperature and ends up at 2100 with no particular unpleasantness. But Max always assumes no effect of adding CO2 when telling us how the future is going to be cool.

One man’s trend is another man’s cherry picking.
Obviously the 1979 starting point for trend making is suspect.
Especially since we are now seeing the presumed post 79 trend flatten when other than GISS data is included.

Max makes a completely valid point and as good as any other specualtion with increased CO2 assumptions. CO2 has been soaring and temperature not. Any assumptions that increased CO2 is certain to trend temperature back on AGW track is no beter specualtion than Max’s.

#163 John, there is something else which may actually mask a warming signal, despite not picking it up at times on the surface, I take yesterday as example and compare Density Weighted Temperatures from 3 significantly different locations. Forth Worth Texas scorching hot +38.8 C evening surface temp in its Upper air had a DWT to tropopause of 269.31 K. Cloudy Maniwaki’s +16 C chilly July weather gave a DWT 0f 262.93 (upper station near Montreal). Arctic Resolute Bay (central Canadian Archipelago) warmish +11 C gave a DWT of 261.73 K. Subtract surface temperatures from Fort Worth +38.8 C with Resolute +11 C and you get 27.8 K difference on the surface, yet for the entire Troposphere the difference is about 8 K. Thus it is hard to say from the surface record alone what is going on temperature wise, without considering the entire atmosphere.

Over at the open Mind blog, Mr. Galasyn posted a reference to Saltzman and Verbitsky, Paleooceanography, v9, 6, pp767-779, 1994. Briefly, he analyses the climate trajectory in the phase space of 3 variables, ocean temperature, ice mass, and CO2 concentration for the last 200KYr. We have longer and more data now, and I would welcome pointers to later work on the subject.

“This overshoot is in the process of radiatively dissipating, and the climate will return to its earlier defined, greenhouse gas-forced warming signal.”

If, as others have said, there is a committed warming (“heat in the pipeline”) of about half a degree, then why or how would the overshoot dissipate?

[Response: The warming in the pipeline doesn’t represent some pool of heat that is being gradually released into the atmosphere; it represents a radiative imbalance that is being gradually erased as the ocean warms up and comes into equilibrium. An event like a strong El Nino which temporarily pumps some extra heat from the ocean into the atmosphere (and thence to space) can cause a trend that exceeds the rate of gradual approach to equilibrium, whence the system will eventually have to relax back to the underlying trend. Another way of putting it is that a half degree of committed warming spread over, say, 20 years amounts to a trend of .025C per year (which doesn’t count the continued increase of CO2). Anything that exceeds that rate can relax “back” despite the gradual weakening of the temporary ocean heat sink. –raypierre]

Actually, what I find interesting in the last month or two of data is the disconnect between very warm surface temps–as noted by Wayne–and the UAH lower trop anomaly–which is very much in the “average” range. Played around a bit on Woodfortrees to see if I could spot anything meaningful connected with this observation, and thought there might be some hint that this is something that can happen more often during PDO transitions. But that’s just eyeballing.

Any insight on this out there? Or is this just a random fluctuation with no significance?

It is more interesting than digging up old stuff that has little or nothing to do with the issue being discussed here.

There is no doubt that warming will eventually resume. The question that the lead article has raised is whether the current warming interruption will last ten, twenty or forty years.

It appears that the jury is out on this question, as I tried to point out.

I would like to see some reasonable estimates on this, based not only on a relatively short-term myopic fixation on AGW greenhouse gases, but including all the many other factors that influence our climate long-term, as Dr. RayPierre has also pointed out.

Following the long-term trend line is, indeed, a speculation, as Howard S. has written, but, in the absence of another more credible speculation, it remains a viable guess on what is really going to happen longer term.

Bring us a better speculation, along with supporting evidence (not just model outputs), if you can.

#173, Kevin, I lament not seeing DWT maps readily available on the internet. But, from what I gather, the Arctic is as warm as it gets right now. Despite an impressive consistent cloud cover, so now is the time when the ice will disappear very fast. As far MU upper air averages, I think they lack verification, I suspect that they are having problems with accuracy. The data seems unclear, what altitude for comparison sakes (I suspect its roughly defined)? So I take this data with a grain of salt. The Fort Worth tropopause was extremely cold compared to Resolute Bay high up in the Arctic. If MU mixes high troposphere data with mid troposphere, in the case of Fort Worth then it would inject the cold Upper Air (as prescribed by models when there is a very hot surface), conversely so for Resolute Bay’s warm atmosphere. DWT’s from Upper Air balloon profiles demolish all these problems, and give data well within where Greenhouse gases do their thing.

“As such, it [the IPCC report] is full of agenda driven pseudo-scientific exaggerations and distortions, which all go in the direction of making its ‘pitch’.”

“The belief that ‘scientists know best’ is flawed (after all, ‘climatology’ is still in its infancy as a ‘science’ and many of the so-called climate “scientists” are in fact only computer jockeys.”

I congratulate you for allowing us to follow your rhetoric over time and space, very few others do that. That being said, you can’t escape the history you’ve created. You give the appearance not only of being prone to emitting inflammatory nonspecific rhetoric, but also that of a hypocrite.

I don’t know about interrupted warming, but the latest SST data shows a big jump in the last couple months, to levels not seen since 1998; GISS also has June as the second warmest on record (LOTI). That seems pretty significant in light of the still relatively low Nino 3.4 anomalies, suggesting that an even larger increase is likely, though some of it could also be rebound from the recent La Nina (seems too large though and coincides more with the development of El Nino conditions, then again I always hear that anomalies lag ENSO by 3-6 months).

#172 Yes I understand that committed warming relates to a radiative imbalance. But if the El Niño somehow caused a temporary acceleration in the warming, wouldn’t this merely overcome part of the imbalance meaning that warming ought to continue. Perhaps I’m being overly fussy about a throwaway line, but I guess it seems more likely that the current “cooling” is just another expression of the variability.

178 I disagree with the technique of leaving out 1998-2008 when determining the trend. It assumes that the atmosphere and top couple feet of surface can hold enough heat to warm the planet above the background for 20-25 years regardless of negative ENSOs.

I agree that it places a lower bound on the situation, but it isn’t the median expected value. It’s a dangerous position to take, betting on the lower bound. I can be convinced, but so far the hypothesis seems weak.

“An event like a strong El Nino which temporarily pumps some extra heat from the ocean into the atmosphere (and thence to space) can cause a trend that exceeds the rate of gradual approach to equilibrium, whence the system will eventually have to relax back to the underlying trend”

Would you elaborate? The idea that there are elasticities in the climate system makes sense, I just don’t know where to start learning.

Back in 2003, Klyashtorin & Lyubushin demonstrated that there appears to be a superimposed cycle of around 62 years over the underlying trend. Others have put it at around 60 years:
The underlying cause is unknown, just as with various ocean oscillations.
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Doug Bostrom & Hank Roberts; can you see something of potential to ridicule in the name ’Lyubushin’?
Have fun!….. no need to confuse us with any scientific comments.

“But why do so many scientists and political leaders plus many in the media support the man-made global warming theory?”

You answered yourself and whatever credulous bystanders (at an Accuweather blog) were in earshot with these assertions:

‘It’s driven by an estimated 2.5 to 4 billion dollars per year in climate research grants, with the grants going selectively to those scientists who make the most disastrous predictions.’

and

‘The politicians and bureaucrats love the idea of “carbon taxes”, higher taxes on fossil fuels, “carbon footprint offset” schemes, etc., because it gives them more money to spread around (and more power).’

As we’re putatively discussing science and facts here, and in your currently displayed persona you’ve requested that we stick to specifics, are you able to cite some reasonably trustworthy (ie, peer-reviewed or non-partisan governmental) sources for those remarkable claims?

180 Steve asks, ” if the El Niño somehow caused a temporary acceleration in the warming, wouldn’t this merely overcome part of the imbalance meaning that warming ought to continue.”

Not quite. The underlying hypothesis is that the air temp gets ahead of ocean heat content, so air temp pauses as it waits for the oceans to catch up. I can see that for a year or so, but the time scales for Warming, Interrupted seem too long. I’d think the next negative ENSO would get us back to the underlying trend.

NOTE TO MODERATORS: This lack of captcha is GREAT! If it doesn’t attract spam, it would be nice if it could stay this way.

“Point is, when the temperatures are not doing “what they are meant to do” it is hard to deal with sceptics who suggest that within climate variability there is a hidden downward trend.”
“Following the long-term trend line is, indeed, a speculation, as Howard S. has written, but, in the absence of another more credible speculation, it remains a viable guess on what is really going to happen longer term.”
Despite the apparent pause in the atmospheric temperature trend, there is a very NOT hidden downward trend in ice; arctic summer ice cover, glacier mass, increasing Greenland yearly ice melt, and a march of disintegrating ice shelves further south around Antarctica. The denialosphere saying “the warming has stopped” in the face of accelerating ice melt is insane, especially after all the flack they’ve put out about how GISS, HadCRUT & so on are not reliable or accurate. Maybe the UHI effect is decreasing as the average SEER of air conditioning increases; anybody want to bet there’s a correlation? &;>)
My viable guess, looking at the trends in ice, and absent any credible explanation (I don’t think GCRs, or clouds, are melting the ice) is that seashore property isn’t a good longterm investment.

But the author’s math is not grounded in a single physical principal. It is an analysis of a single dependent variable at an intermediate level of the total system and is not isolated from influences from the other components. The entire set of results could just be artifacts of other, un-named, processes.

I fail to see what predictive value there is in this approach.

There is little, if any, predictive value in this approach, but there is a lot of *proscriptive* value (GHG’s should be proscribed lower)

This contrasts with highly valuable 3 to 10 day ahead weather predictions, and ENSO forecasts, both of which have highly tangible values.

Things like how many years before my house gets inundated etc. is still an intangible risk, even if one knows the risk is increasing gradually.

All long term climate predictions can just tell you that you should reduce ghg’s, not how best to deal with the consequences.

I have a problem with your response 172.
You say that an El Niño pump heat from ocean towards space.
In this case the unbalance should be negative (increasing of flux towards space).
But, in this link :http://earthobservatory.nasa.gov/Features/OceanCooling/page2.php I see, on the contrary, an positive net TOA flux.
There is, as it is normal in this case, a correlation with the increasing ocean heating.
Can you explain me?

“… In the little ice age, as I understand it, temperatures fell gradually over a number of centuries to correspond in some as yet unexplained way with the “Maunder Minimum,” a period of reduced sunspot activity. It seems fairly reasonable to suppose that had humans emitted no CO2 at all, temperatures would have risen again anyway in the same pattern that they had fallen previously, i.e. gradually and over a number of centuries…

I would like to add that there is a big long-term experiment ongoing at CERN, that is checking out some initial work by Svensmark, indicating that the climate effects resulting from the sun’s activity are far more complicated than just the simple emission intensity of sunlight. For instance, the low sunspot activity during the “Maunder Minimum”, as mentioned by Alex may well have complex effects other than the purely thermal from sunlight.
And then of course, there has been a long and remarkable dearth of sunspots of late, and debate as to when the next solar cycle will start. There was stir of excitement in some quarters a couple of weeks ago, when a pair of modest sunspots were noticed. However, the SOHO MDI real-time image seems to have returned to the long-time norm of zero sunspots. Check it out for yourself at:http://sohowww.nascom.nasa.gov/data/realtime/mdi_igr/512/
But then, if there are no sunspots, and if you are concerned about global warming, then clearly, it is GOOD NEWS of indication of at least more delayed warming. (not to mention less interference in communications ETC)
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BTW, a quickie to; Kevin McKinney, Hank Roberts, and Patrick 027;
Please look carefully at the high quality image from SOHO MDI, and compare it with human capability on the ground in protected looking for limb darkening of the sun. Oh and BTW, compare also with the Wiki’ article and its poor quality exaggerated image. Oh and BTW Wiki’ does not mention a second and possibly more important reason for this trivial nit picking consideration.

Something of a perfect storm is developing in the denial world, as can be seen from comments here http://www.skepticalscience.com/Climate-time-lag.html and here http://www.huffingtonpost.com/david-horton/like-a-diamond-in-the-sky_b_234611.html and from post and comments here http://www.abc.net.au/unleashed/stories/s2626711.htm. The elements are Plimer’s book, some wacky theories on cosmic rays and clouds and time lags issuing from denial central, and an absolute renewed refusal to recognise any changes in ice cover, ecology, ocean chemistry etc, and indeed a refusal to recognise even the greenhouse effect itself. And sucked into the vortex, dear Real Climatians, is this very post “Warming, interrupted: Much ado about natural variability”, seen as RC’s endorsement of a downward turn in temp rise, and final proof that CO2 is irrelevant to the world and just a plot by Gore and Soros to rule us all. It is all reminiscent, is it not, of rolling the boulder up the hill over and over while crows peck out your entrails, or whatever that marvelous mythological model is.

Interesting read. But seems like a real problem for prospects of geo-engineering – if there are such non-linear (or unpredictable) effects from ocean etc then controlling geo-engineering interventions is going to be difficult. …

Hank Roberts #165 “Max Acker (manacker) draws a trend from 1850 forward in temperature and ends up at 2100 with no particular unpleasantness. But Max always assumes no effect of adding CO2 when telling us how the future is going to be cool.”

But manacker’s trend is (so far) empirically observed. He assumes nothing. He merely extrapolates this (linear) trend. Any CO2 increase is already included as a contributor to the observed temperature trend since the Industrial Revolution.

Now a reasonable person might think this a point worth making.

And yet you immediately follow this post by saying (#166) “Look [manacker] up. Why bother replying? Look what he posts about RC and climate” so enjoining us to ignore manacker on account of his political views.

When we think of 2 degrees we kinda forget the issue of the burst of GHG emissions that, after a lag of a few hundred years, followed the up-kick in temperature in past deglaciations. Estimates of The Lag range from about 800 years to much less – maybe as short as 200 years.

For longer lags (C800 years) the source of CO2 is sometimes suggested as being the CO2-laden waters of the deep ocean, 800 years being about the over-turning time for the ocean.

I haven’t been able to find any clear source for the volume of CO2 and CH4 that would give rise to the observed changes for shorter lags, but that it happened is undeniable.

As far as I can find the rate of temperature rise over these warming periods is maybe one degree K per thousand years – i.e. an average of about 0.1 degrees per century. Thus the likely temperature change to trigger the release of CO2 and CH4 is of the order of 0.2 to 0.8 degrees K.

We have already measured 0.6 degrees warming attributable to man over the last century, so it seems we are well into the band of temperature rise that triggered the GHG release at the end of The Lag in the past. So just how close to disaster are we?

Hank Roberts #165 “Max Acker (manacker) draws a trend from 1850 forward in temperature and ends up at 2100 with no particular unpleasantness. But Max always assumes no effect of adding CO2 when telling us how the future is going to be cool.”

But manacker’s trend is (so far) empirically observed. He assumes nothing. He merely extrapolates this (linear) trend. Any CO2 increase is already included as a contributor to the observed temperature trend since the Industrial Revolution.

Now a reasonable person might think this a point worth making.

I think I’m a reasonable person, and my understanding is that a higher concentration of atmospheric CO2 means a higher *rate* of warming – a steeper trend – and that since atmospheric CO2 is steadily increasing, we should expect to see the slope of the warming trend steadily increasing too.

[Response: No, this has been covered here before. The radiative forcing by CO2 is roughly logarithmic. That means that an exponential increase in CO2 leads to a roughly linear (i.e. constant) warming trend. Of course, this is somewhat irrelevant anyway, since the existence of other significant anthropogenic forcings such as the other greenhouse gases, sulphate aerosols, land use change, etc. leads to a far more complicated time history of anthropogenic forcing. There is also a substantial contribution from internal variability which adds considerable ‘noise’ to the record. That means that it is extremely difficult if not impossible to isolate the the response of the climate to CO2 increases alone in the observed trends. If that were possible, we’d have a tight constraint on climate sensitivity from the observational record alone. But we don’t. We need to turn to other sources of information, e.g. the observed paleoclimate response to past changes, to place better constraints on the ‘climate sensitivity’. -mike]